The present invention relates to a semiconductor thin film made from a compound semiconductor typically composed of a group III nitride, a semiconductor element using the semiconductor thin film, and a semiconductor device using the semiconductor element, and fabrication methods thereof.
In recent years, semiconductor light emitting devices, such as a semiconductor laser or a light emitting diode (LED), enabling light emission in a range from a visible light region to an ultraviolet light region by using group III nitride based compound semiconductors such as AlGaInN have been actively developed. In particular, in the optical recording field, a semiconductor laser capable of emitting light in a short-wavelength region has been required to be practically used for improving the recording density of optical disks or the like.
Recently, an AlGaInN based semiconductor laser capable of realizing continuous oscillation for 300 hr at room temperature has been reported in Jpn. J. Appl. Phys. 35L74 (1996); and ditto 36L1059 (1997), in which a semiconductor layer made from a group III nitride based compound semiconductor is grown on a substrate made from sapphire via a buffer layer made from gallium nitride (GaN) by a metal organic chemical vapor deposition (MOCVD) method.
The above-described semiconductor laser, however, has a problem. As is apparent from a curve showing a change in drive voltage applied to the semiconductor laser with elapsed time, the drive voltage is gradually increased from the initial current-carrying period. This means that the voltage characteristic is gradually degraded with elapsed time. The degradation of the voltage characteristic may be dependent on the fact that the group III nitride based compound semiconductor layer formed on the substrate has threading-dislocations (which are defects propagated to pass through the crystal) at a density ranging from about 1xc3x97108 /cm2 to about 1xc3x97109 /cm2.
Accordingly, to realize the practical life time of the semiconductor laser for 10,000 hr or more, it is required to reduce the density of the threading-dislocations, and to meet such a requirement, various methods have been examined.
One method has been proposed in Jpn. J. Appl. Phys. 36 L899 (1997) and Appl. Phys. Lett., 71, 2638 (1997), in which a GaN underlying layer is formed on a sapphire substrate via a buffer layer, a mask layer made from silicon dioxide (SiO2) and having a periodical pattern of stripes (width; 1 to 4 xcexcm) arranged with a pitch of 7 xcexcm is stacked on the GaN layer; and a GaN layer is grown on the mask layer selectively in the lateral direction by a halide vapor-deposition method or MOCVD method. In the case of adopting the method of growing the GaN semiconductor layer on the mask layer in the lateral direction by selectively growing GaN from the underlying GaN layer exposed from openings between the periodical stripes of the mask layers, the density of threading-dislocations in the GaN layer on the SiO2 mask layer can be reduced to about 1xc3x97107 /cm2.
As is reported in Journal of Crystal Growth, vol. 189-190 P. 820-5 (1998), an AlGaInN based semiconductor laser diodes formed on the semiconductor layer prepared by adopting the above method can realize the practical life time of 1150 hr or more.
Incidentally, with respect to the semiconductor layer formed by selective growth using the above-described mask layer, it has been revealed by the present inventors that a deviation in crystal orientation (c-axis), which is in the order of 0.4-0.5xc2x0, occurs between a portion of the semiconductor layer on the stripe of the mask layer and a portion of the semiconductor layer in the opening of the mask layer.
If a semiconductor element is formed on the semiconductor layer having such a deviation in crystal orientation, an active region of the semiconductor element contains the deviation in crystal plane. As a result, various characteristics of the semiconductor element are degraded, and if the semiconductor element is configured as a semiconductor laser, light emission efficiency and life time are reduced.
The problem of the conventional method above mentioned will be more fully described. A substrate for growth of a GaN based semiconductor, which is made from sapphire or SiC as described above, is largely different from the GaN based semiconductor in lattice constant and thermal expansion coefficient, and consequently, if the GaN based semiconductor is directly grown on the substrate, defects such as dislocations occur in the growth layer, with a result that it is difficult to grow a high quality single crystal of the GaN based semiconductor epitaxial layer.
For this reason, as described above, an underlying GaN layer containing high density threading-dislocations, which is formed on the sapphire substrate or SiC substrate via a buffer layer; a mask layer made from SiO2 and being patterned into stripes arranged with a specific pitch is formed on the underlying GaN layer; and GaN is grown from the underlying GaN layer exposed through openings between the stripes of the mask layer selectively in the lateral direction, to thereby form the GaN semiconductor containing defects at a low density on the mask layer. However, as a result of analyzing the sample thus prepared by electron diffraction or X-ray diffraction, it was proved that a deviation in crystal orientation, which is in the order of 0.4-0.5xc2x0, occurs between a portion of the GaN semiconductor layer formed on the stripe of the mask layer and a portion of the GaN semiconductor layer formed in the opening of the mask layer.
The reason why there occurs a deviation in crystal orientation (c-axis) is that upon lateral growth of GaN on the SiO2 mask layer, there occurs a deviation in crystal growth direction between a portion on the stripe of the mask layer and a portion in the opening of the mask layer.
Based on the structural analysis by the present inventors using transmission electron microscopy or X-ray diffraction method, it became evident that if the orientation of the stripes of the SiO2 mask is set to the  less than 11-20 greater than  direction and a deviation in the crystal growth direction occurs along the  less than 11-20 greater than  direction, crystal defects are induced; but if the orientation of the stripes of the SiO2 mask is set to the  less than 1-100 greater than  direction and a deviation in the crystal growth direction occurs along the  less than 1-100 greater than  direction, crystal defects are not induced. In each case, however, there occurs a deviation in crystal growth orientation mainly depending on a difference in thermal expansion coefficient between SiO2 as the mask material and GaN. As a result, if the SiO2 mask layer is not present in the lateral growth of GaN, it is possible to suppress the deviation in crystal growth orientation.
However, since the SiO2 mask layer is provided for removing threading-dislocations from the substrate side to the semiconductor layer, removal of the mask layer is incompatible with the purpose of reducing the defect density of the semiconductor layer.
An object of the present invention is to provide a semiconductor thin film capable of reducing the density of threading-dislocations and also suppressing occurrence of a deviation in crystal orientation, a semiconductor element using the semiconductor thin film, and a semiconductor device using the semiconductor element. Another object of the present invention is to provide methods of fabricating the above semiconductor thin film, semiconductor element, and semiconductor device.
The present inventors have found that while in the lateral growth of a semiconductor, threading-dislocations are bent by growth facets generated accompanied by the growth, such bending of the threading-dislocations can be performed not only by the above growth facets but also by facets artificially formed, and on the basis of the knowledge, the present inventors have accomplished a semiconductor thin film capable of reducing the density of the threading-dislocations at a specific region and also suppressing occurrence of a deviation in crystal orientation, and a semiconductor element using the semiconductor thin film, and a semiconductor device using the semiconductor element, and fabrication methods thereof.
According to a first aspect of the present invention, there is provided a semiconductor thin film including an underlying semiconductor layer in which a plurality of facets are arranged, and a selectively grown/buried semiconductor layer formed to cover the underlying semiconductor layer, wherein the facets of the underlying semiconductor layer are formed by planes tilted with respect to the disposition plane of the underlying semiconductor layer.
According to a second aspect of the present invention, there is provided a semiconductor element including a semiconductor thin film having an underlying semiconductor layer in which a plurality of facets are arranged and a selectively grown/buried semiconductor layer formed to cover the underlying semiconductor layer, and a semiconductor element main body formed on the semiconductor thin film, that is, on the selectively grown/buried semiconductor layer or on a semiconductor layer formed thereon, wherein the facets are formed by planes tilted with respect to the disposition plane of the underlying semiconductor layer.
According to a third aspect of the present invention, there is provided a semiconductor device including: a semiconductor element including a semiconductor thin film and a semiconductor element main body formed on the semiconductor thin film, the semiconductor thin film having an underlying semiconductor layer in which a plurality of facets are arranged and a selectively grown/buried semiconductor layer formed to cover the underlying semiconductor layer; wherein the facets are formed by planes tilted with respect to the disposition plane of the underlying semiconductor layer. Concretely, the semiconductor device of the present invention may be a single semiconductor device using the semiconductor element or a semiconductor integrated circuit system using a plurality of the semiconductor elements.
According to a fourth aspect of the present invention, there are provided methods of fabricating the above-described semiconductor thin film, semiconductor element and semiconductor device, each method including the steps of forming an underlying semiconductor layer on a substrate in such a manner that a plurality of facets are arranged in the underlying semiconductor layer, and growing a selectively grown/buried semiconductor layer in such a manner that the selectively grown/buried semiconductor layer covers the underlying semiconductor layer, wherein the facets are formed by planes tilted with respect to the disposition plane of the underlying semiconductor layer.
In accordance with one preferred embodiment of the fabrication method of the present invention, the facets of the underlying semiconductor layer are artificially formed, and the selectively grown/buried semiconductor layer is epitaxially grown on the underlying semiconductor layer.
In accordance with another preferred embodiment of the fabrication method, a mask is formed on a plane of the substrate on which the underlying semiconductor layer is to be formed; the underlying semiconductor layer having the facets is formed by selective growth, so-called ELO (Epitaxy Laterally Overgrowth), and the mask is removed; and the selectively grown/buried semiconductor layer is epitaxially grown.
As described above, according to the semiconductor thin film, the semiconductor element, and the semiconductor device, and the fabrication methods thereof of the present invention, the selectively growth/buried semiconductor layer is selectively grown from the facets, that is, tilting planes of the underlying semiconductor layer. With this configuration, threading-dislocations are formed in the selectively grown/buried semiconductor layer in such a manner that each of the threading-dislocations laterally extends, that is, bendingly extends from one of the facets of the underlying semiconductor layer in the direction substantially along the disposition plane of the underlying semiconductor layer, being joined to another of the threading-dislocations bendingly extending from the opposed one of the facets, and bendingly extends from the joined portion in the direction crossing the disposition plane of the underlying semiconductor layer. As a result, low defect density regions in which the threading-dislocations are little present can be formed at portions other than the above joined portions of the threading-dislocations in the selectively grown/buried semiconductor layer.
Further, according to the present invention, since the configuration in which the mask made from SiO2 or the like is buried is not adopted, it is possible to avoid occurrence of the above-described deviation in crystal orientation (C-axis).
According to the semiconductor element of the present invention and the semiconductor device using the semiconductor element, since an active region (operational region) of the semiconductor element is formed on a low defect density region of the selectively grown/buried semiconductor layer or on a semiconductor layer formed thereon, it is possible to enhance the characteristics of the semiconductor element.
It should be noted that the facet of the present invention means not only a perfect tilting plane but also a plane which is somewhat curved partially or entirely and which has a principal plane tilted at a specific angle xcex1.